DESCRIPTION: In studying the regulation of the stress activated kinases (JNKs) in cells maintained under normal growth conditions, the applicant identified a 23 kD protein which inhibits JNK activities through its association with the JNK-jun complex (but not with the individual components). Microsequencing of this biochemically purified JNK inhibitor identified glutathione-S-transferase pi as the inhibitory component. The finding of GSTpi as the JNK inhibitor was confirmed through in vitro and in vivo experiments using purified GSTpi, its respective cDNA, and specific GSTpi inhibitors. In recognizing the link between changes in redox potential and GSTpi, the applicant hypothesizes that, by means of its response to cellular redox potential and its regulation of JNK activity, GST pi is a key regulator of the cell s response to stress. This hypothesis is to be tested by comprehensively characterizing the GSTpi activities as the JNK inhibitor, and by elucidating GSTpi s effect on cellular phenotypes such as drug resistance or apoptosis via its regulation of JNK signaling. Specifically, the applicant will 1) characterize the nature of the GSTpi inhibition of JNK; 2) identify peptides that can modulate GSTpi as the JNK inhibitor; 3) characterize GSTpi as a sensor of redox changes in JNK signaling; 4) determine, using selective peptides identified in earlier aims, the ability to alter apoptosis and drug resistance in MCF-7 breast cancer cells that express low levels of GSTpi and in adriamycin-resistant MCF-7 cells that express 20-30 fold higher levels of GSTpi; and 5) determine the specificity of GSTpi inhibitory activity for other kinases involved in the stress response. Overall, the proposed studies will provide novel therapeutic targets based on new understanding of JNK regulation by GSTpi and its implications regarding key cellular end points, including apoptosis and drug resistance.